HUMAN FATTY-ACID SYNTHASE - PROPERTIES AND MOLECULAR-CLONING

Fatty acid synthase (FAS; EC 2.3.1.85) was purified to near homogeneit y from a human hepatoma cell line, HepG2. The HepG2 FAS has a specific activity of 600 nmol of NADPH oxidized per min per mg, which is about half that of chicken liver FAS. All the partial activities of human F AS are comparable to those of other animal FASs, except for the beta-k etoacyl synthase, whose significantly lower activity is attributable t o the low 4'-phosphopantetheine content of HepG2 FAS. We cloned the hu man brain FAS cDNA. The cDNA sequence has an open reading frame of 751 2 bp that encodes 2504 amino acids (M(r), 272,516). The amino acid seq uence of the human FAS has 79% and 63% identity, respectively, with th e sequences of the rat and chicken enzymes. Northern analysis revealed that human FAS mRNA was about 9.3 kb in size and that its level varie d among human tissues, with brain, lung, and liver tissues showing pro minent expression. The nucleotide sequence of a segment of the HepG2 F AS cDNA (bases 2327-3964) was identical to that of the cDNA from norma l human liver and brain tissues, except for a 53-bp sequence (bases 38 99-3944) that does not alter the reading frame. This altered sequence is also present in HepG2 genomic DNA. The origin and significance of t his sequence variance in the HepG2 FAS gene are unclear, but the varia nce apparently does not contribute to the lower activity of HepG2 FAS.